Method of operating turbojet and ramjet engines



United States Patent Okla, assigu'ors to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Oct. 23, 1957, Ser. No. 691,789

: 10 Claims. (Cl. 60--35.4)

This invention rel-ates to the operation of turbojet and ramjet engines. In one aspect it relates to a method for stabilizing combustion in the combustion zone of turbojet and ramjetrengines. In another aspect itrelates to a method for stabilizing the flow of liquid turbojet and ramjet engine fuel to the combustion zone of the engine in conduits and injection nozzles exposed to temperature higher than the boiling range of the fuel. In still another aspect this invention relates to amethod for operating turbojet and ramjet engines smoothly and with a of flame blowouts.

It is well known in the jet engine art that variations in the delivery rate of fuel to the jet combustion zones or combustors result in unsteady burning and, in extreme cases, in engine flame blowout. This variation in fuel rate at high temperatures can, in some cases, be attributed to the formation of vapor pocketsor vapor in the fuel line with consequent variation in fuel delivery by the nozzle even while operating at constant pressure. In this type of engine it is impossible to construct the engine in such a manner that the fuel pipes are always in a region of low temperature. Therefore, because the fuel pipes at their points of entry into the combustion chambers are regions of high temperature, premature vaporization'of the fuel frequently occurs. When bubbles of vaporous fuel form on the walls of the fuel pipes, combustion instability is the obvious result because at the instant a large bubble of vapor is injected into the combustion zonethe rate of fuel introduction is relatively small in comparison to the rate of fuel introduction when liquid is introduced into the combustion chamber. The passage of'fuel vapor into a ramjet or turbojet combustion engine designed for liquid fuel injection is very similar in result to that which occurs when an automobile engine vapor locks.

An object of our invention is to provide a method for improving combustion stability in turbojet and ramjet engines.

Another object of our invention is to provide a simple and an inexpensive method for increasing combustion stability in such engines.

Still another object of our invention is to provide a method for minimizing fuel vaporization or vapor locking in the fuel line in high temperature areas adjacent the combustion zone of the engine.

Still other objects and advantages will be. realized upon reading the following description which fully describes our invention.

One factor which affects bubble formation in fuel con duits of jet engines under high temperature conditions is surface tension of the liquid fuel. When the boiling liquid does not wet the surface of the fuel conduit bubbles formed are large because the vapor is formed in direct contact with the dry surface of the conduit and under this condition heat transfer through the wall of the conduit to the vapor is large. The vapor formed in direct contact with the dry conduit is superheated and the size of the bubble grows rapidly. Upon detachment of the bubble from the wall of the conduit, as caused by the flow of the liquid fuel in the conduit under pressure, the bubble continues to grow because it has been superheated by direct contact with the walls of the conduit. This superheated bubble of vapor vaporizes an additional 'ice 2. amount of fuel from the wall of the bubble thereby making. the bubble still larger. A time may be reached at which the diameter of the bubble of vaporized fuel is equal to the diameter of the fuel conduit and, in this condition, when this bubble of fuel flows through the nozzle into the combustion chamber, there is a moment of vapor lock. Frequent passage of such quantities of vapor through the nozzle into the combustion chamber causes unstable and rough combustion.

We have found upon the addition of an oil-soluble surface-active agent to the liquid fuel that the fuel wets the walls of thefuel conduit even in close proximity to the combustion chamber. When the fuel easily wets the walls of the fuel conduit, even though a bubble of vapor forms, it is not superheated because the surface of the conduit is wet with a film of liquid fuel. Since the bubble of vapor is not superheated its size is considerably smallerthan the size of a bubble formed in direct contact with the walls of the conduit. Thus, upon addition of our surface-active agent to the fuel the size of the bubbles formed in the high temperature region of the fuel conduits are considerably smaller in diameter than the diameter of the pipe. There is, accordingly, no vapor lock. Furthermore, the presence of a surface act-ive agent in the fuel is believed to create a diffusion barrier around the bubble through which fuel vapor must pass in order to enlarge the bub-ble and this diffusion barrier is believed to restrict vapor flow into the bubble. We have found that upon addition of a small amount of an alkyl pdlysilonane having the following formula markedly increases the stability of combustion in such jet engines,

'I he alkyl polysiloxane is a polymer in which n is an integer varying from 2m 50, preferably from 3 to 20. The alkyl groups illustrated by R R R R R and R represent alkyl groups containing from 1 to 3. carbons per alkyl group. These alkyl groups need not all be the same. A very useful and preferred additive is one in which all of the alkyl groups are methyl groups.

Vapor locking of fuel in fuel flow conduits is particularly bad at low fuel flow rates because the fuel is exposed to the high temperature for longer periods of time than fuel flowing at high fuel rates, We have found upon addition of an alkyl polysiloxane that the fuel rate to a jet engine combustor is decreased by a factor of ten before the eng-ine flame blows out.

The fuel used according to the example illustrative of this invention boils in the range of about F. to 600 F., the specifications for jet fuels being well known. Oommon jet fuels are LIP-3, JP-4, JP-. 5 fuels which conform to specific well known specifications. Kerosenes, distillate oils and even gasoline boiling range hydrocarbons are used as jet engine fuels. Such fuels are usually easily available. The alkyl polysiloxane is added to such fuels in amounts ranging from about 0.2 to 0.001 weight percent, usually in -an amount from about 0.05 to about 0.002 weight percent. When practicing this invention the above-mentioned alkyl polysiloxane is added to the jet fuels in the above-mentioned amounts and, in one case, was 0.01 Weight percent methyl polysilox-ane (-all alkyl groups being methyl groups).

Comparative runs were made in [a jet microburner using normal heptane as a fuel. The fuel nozzle tempera ture in each run was 575 F. In the first run normal heptane was used as the fuel and the combustion became erratic and unstable when't-he fuel rate was reduced to 0.03 pound per hour. Instability was evident and low In a second run the same normal heptane was used as fuel but to this fuel was added 0.01 weight percent methyl polysil-oxane, corresponding to the above-given formula, in which all of the alkyl groups were methyl groups and the average molecular weight was such that n was approximately 7. The term approximately 7 means that the methyl polysiloxane, or methyl siloxane polymer material used was a mixture of methyl sil'onane polymers, in all of which the n values were whole numbers. In commercially available material there are, for example, some molecules in which n is 8, and many molecules in which n is 7, so that the average 11 for the composition is approximately 7. This additive reduced bubble formation in the fuel to such as extent that the fuel flow was reduced to 0.003 pound per hour before the flame became unstable and blowout occurred. The instability of the flame in this run was much less violent but of higher frequency variations than in the first-mentioned run. The instability being less violent is believed to be caused by the presence of a larger number of smaller bubbles than formed in the fuel in the absence of our additive. Apparently none of the bubbles were sufficiently large to cause flame blowout until the fuel rate had been reduced about tenfiold.

While certain embodiments of the invention have been described for illustrative purposes, the invention obviously is not limited thereto.

We claim:

1. A method for feeding 'a liquid hydrocarbon jet engine fuel at a uniform liquid flow rate to turbojet and to namjet engine combustion chambers in conduits exposed to temperatures higher than the boiling range of the fuel, said method comprising incorporating into said fuel a small but suificient amount of an alkyl polysilonane soluble in said fuel to produce uniform liquid flow through said conduits, said alkyl polysiloxane having the genenal formula:

in which R R R R R and R are alkyl groups containing from 1 to 3 carbon atoms per alkyl group, n is an integer from 2 to 50, and passing said fuel containing said alkyl polysiloxane through said conduits into said combustion chambers.

2. The method of claim 1 wherein from about 0.2 to about 0.001 percent by weight of said alkyl polysilonane is incorporated into said fuel.

3. The method of claim 2 wherein said jet fuel possesses a boiling range between about and 600 F.

4. The method of claim 2 wherein said alkyl groups are methyl groups.

5. The method of claim 2 wherein n is approximately 7.

6. A method for continuously operating turbojet and ramjet engines in the absence of flame blowouts due to vapor formation in fuel conduits of said engines which are exposed to temperatures above the boiling range of the liquid hydrocarbon jet fuel, said method comprising incorporating into said fuel a small but sufficient amount of an alkyl polysiloxane soluble in said fuel to produce uniform liquid flow through said fuel conduits, said alkyl polysiloxane having the general formula:

R3 II in which R R R R R and R are alkyl groups containing from 1 to 3 carbon atoms per alkyl group, n is an integer from 2 to 50, passing said fuel containing said alkyl plolysiioxane through fuel conduits exposed to said temperatures and thence into the combustion zones of said engines, and burning said fuel in said combustion zones with the production of thrust.

7. The method of claim 6 wherein from about 0.2 to about 0.001 percent by weight of said alkyl polysilox an'e is incorporated into said fuel.

8. The method of claim 7 wherein said jet fuel possesses a boiling range between about 100 and 600 F.

9. The method of claim 7 wherein said alkyl groups are methyl groups.

10. The method of claim 7 wherein n is approximately 7.

References Cited in the file of this patent UNITED STATES PATENTS 2,432,109 Zisman et I211. Dec. 9, 1947 2,529,496 Hughes et a1 Nov. 14, 1950 2,563,305 Britton et al. Aug. 7, 1951 

1. A METHOD FOR FEEDING A LIQUID HYDROCARBON JET ENGINE FUEL AT A UNIFORM LIQUID FLOW RATE TURBOJET AND TO RAMJET ENGINE COMBUSTION CHAMBERS IN CONDUIT EXPOSED TO TEMPERATURES HIGHER THAN THE BOILING RANGE OF THE FUEL, SAID METHOD COMPRISING INCORORATING INTO SAID FUEL A SMALL BUT SUFFICIENT AMOUNT OF AN ALKYL POYSILOXANE SOLUBLE IN SAID FUEL TO PRODUCE UNIFORM LIQUID FLOW THROUGH SAID CONDUITS, SAID ALKYL POLYSILOXANE HAVING THE GENERAL FORMULA: 